In the process for electrolytic production of aluminium, such as by the Hall-Heroult process where aluminium is produced by reducing aluminium oxide in an electrolytic smelting pot filled with melted electrolyte in the form of a fluoride-containing mineral to which aluminium oxide is supplied, the process effluent gases comprise fluoride-containing substances such as hydrogen fluoride (HF) and fluorine containing dust. As these substances are extremely damaging to the environment, such must be separated before the process effluent gases may be discharged into the surrounding atmosphere. At the same time, the fluorine-containing melt is essential to the electrolytic process, and thus it is desirable to recover the compounds for recirculation to the electrolysis process. This recirculation may take place by absorption of the fluorine-containing substances on a particulate adsorbent.
As noted, the electrolytic reaction occurring in the electrolytic smelting pots produces process effluent gas in the form of hot, particle-laden effluent gas, typically cleaned in a gas cleaning unit before being discharged to the atmosphere. An example of a gas cleaning unit for cleaning the effluent gas generated in electrolytic smelting pots is disclosed in U.S. Pat. No. 5,885,539. The gas cleaning unit disclosed in U.S. Pat. No. 5,885,539 comprises a first contact reactor and a second contact reactor. The effluent gas from the electrolytic smelting pots is first forwarded to the first contact reactor and is, in the first contact reactor, brought into contact with recycled alumina. The partly cleaned effluent gas is then forwarded to the second contact reactor and is, in the second contact reactor, brought into contact with fresh alumina. The partly used alumina is recycled from the second contact reactor to the first contact reactor. A dust removal device removes the alumina from the effluent gas, which is then discharged to the atmosphere.
The system for recovery of the fluoride compounds comprises a filter system, which is included in a closed system. It is important to have stable transport of the gases from the aluminium production process to the filter system. This transport is accomplished in gas ducts where the gases, by means of large fans, are conveyed through the gas ducts, comprising main ducts and branch ducts, to the filter system. For each aluminium production cell a branch duct is brought into the main duct, the cross section of the main duct increases gradually, by means of diffusers as the gas quantity increases. It is very important for the process as well as the environment that the gas distribution is as even as possible, and traditionally this is achieved by an increasingly stronger throttling of the process gas in the branch duct the closer to the suction fans the branch duct is localized. Throttling represents sheer energy loss through a pressure drop.
Gas cleaning units for cleaning of process gases produced during electrolytic processing of aluminium include both centralized systems and decentralized systems. Centralized systems often include one or several halls comprising electrolytic cells whereby each hall may comprise between 70 and 150 electrolytic cells, with cleaning equipment arranged centrally between the halls or outside. The centralized system is connected with each of the cells by means of comprehensive and costly ductwork. Aluminum oxide used as adsorbent during the cleaning process is stored in separate silos before or after use in the cleaning process and thereafter transported back to each cell by means of transportation vehicles, cranes or other transportation system for aluminum, such as a system for transportation in a compact phase.
Decentralized systems are used to clean process gases from between 5 to 40 electrolytic cells, more preferably between 10 to 20 electrolytic cells. As such, less ductwork is required, and transportation needs for movement of aluminum oxide are greatly reduced. Large flexibility is achieved as to operation start up, and distance between aluminum oxide storage and the electrolytic cells may be minimized. Additional benefits achieved by decentralized systems are described in U.S. Pat. No. 6,406,524.
Although systems for cleaning process effluent gases produced during electrolytic processing of aluminium are known, improved systems that reduce operation costs, reduce equipment footprint, reduce capital costs, and/or increase adaptability to meet specific system requirements for larger production facilities, are still needed in the aluminium production industry.